The present invention relates to a complex lens that consists of a plurality of stacked lens portions, and particularly, relates to the complex lens for a tandem scanning optical system employed in an imaging device such as a color laser printer for converging a plurality of light beams deflected by a deflector. Further, the present invention relates to a manufacturing method of such a complex lens for a tandem scanning optical system.
A tandem scanning optical system employed in a color laser printer is provided with four semiconductor lasers and four photoconductive drums that correspond to colors Y (Yellow), M (Magenta), C (Cyan) and K (blacK), respectively. In such a tandem scanning optical system, it is preferable to make at least one part of the optical system shareable among the colors to downsize the system. The polygon mirror may be shared.
When a polygon mirror is shared, four light beams are incident on the polygon mirror such that they are arranged in an auxiliary scanning direction, which is coincident with a direction of the rotation axis of the polygon mirror. The four light beams deflected by the polygon mirror are converged by an fxcex8 lens and the optical paths thereof are separated by mirrors. The separated four light beams form scanning lines on the respective photoconductive drums.
It is preferable that the four light beams deflected by the polygon mirror are converged by the respective lens elements in order to obtain the most suitable optical performance. On the other hand, the smaller the thickness of the polygon mirror is, the smaller the spaces among the four light beams are in the vicinity of the polygon mirror. This does not allow employing independent lens elements for the respective light beams. Therefore, a lens in the fxcex8 lens arranged close to the polygon mirror should be a complex lens that consists of stacked four lens portions.
It is therefore an object of the invention to provide an improved manufacturing method of a complex lens for a tandem scanning optical system that is capable of reducing the positional error among the lens portions when the complex lens that consists of stacked lens portions is molded as a single-piece element.
For the above object, according to the present invention, there is provided a manufacturing method of a complex lens for a tandem scanning optical system, including a step for preparing molding dies for forming a cavity to unitarily form the complex lens as a single-piece element, and a step for injecting lens material into the cavity. The molding dies include a pair of single-piece unitarily formed mirror surface cores that form a plurality of lens surfaces of the complex lens at an incident side and a plurality of lens surfaces at an exit side, respectively. The complex lens consists of a plurality of stacked lens portions for converging a plurality of light beams, which are modulated independently and deflected by a deflector, onto a surface to be scanned, respectively, for forming a plurality of scanning lines at the same time.
Further, a complex lens for a tandem scanning optical system according to the present invention is formed as a single-piece element that is equivalent to a combination of independent lens portions stacked one on another, the lens surfaces of the lens portions at the incident side are formed by a single-piece mirror surface core and the lens surface of the lens portions at the exit side are formed by another single-piece mirror surface core.
Since the lens surfaces at the incident side and the lens surface at the exit side are formed by the single-piece mirror surface cores, respectively, during the molding process, the relative positional error among the lens surfaces at the incident side and that at the exit side can be reduced.
It is preferable that each of the mirror surface portions of the mirror surface cores has a concave sectional shape in a direction perpendicular to the direction in which a plurality of light beams scan (i.e., an auxiliary scanning direction). That is, the lens surfaces of the molded complex lens preferably have convex sectional shapes in the direction perpendicular to the scanning direction of the light beam.
When the mirror surface portions of the mirror surface core have convex sectional shapes, the boundary of the mirror surface portions will be a valley. Therefore, the boundary portions cannot be sharply processed because of the limitation of a cutting tool, which requires predetermined margins at the boundaries. The margins are not employed as lens surfaces.
On the other hand, when the mirror surface portions of the mirror surface core have concave sectional shapes, the boundary of the mirror surface portions will be a peak. Therefore, since the boundary portions can be sharply processed by the cutting tool, the mirror surface portions can be processed without the margins.
The mirror surface portions of at least one of the mirror surface cores at the incident and exit sides may be formed as rotationally-symmetrical concave surfaces with respect to respective optical axes such as spherical surfaces. In such a case, the lens surfaces of at least one of the incident and exit sides are formed as rotationally-symmetrical convex surfaces with respect to respective optical axes.